Hello asper Van't Veen,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
I understand that you are trying to upgrade Postgres flexible server from 13->16 in West Europe, despite that you've done the same twice in your other subscription without any issues, but this came up with error.
The error that came up could be due to several reasons, so sorry for the inconveniences this might have caused you. Try to understand the followings and review details in the provided links:
- Azure Database for PostgreSQL runs a pre-check procedure to identify potential issues and if any incompatibilities are found or certain extensions are not supported during the upgrade, also if High Availability is enabled, the upgrade aborts with this type of an error - https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-major-version-upgrade
- Check your Network Security Group because, misconfigured NSGs can block necessary traffic and if you have a read replica enabled, you will need to delete it before performing the upgrade on the primary server, this can cause the upgrade to fail - this link is a similar solution discovered by owner - https://learn.microsoft.com/en-us/answers/questions/1538166/failed-to-upgrade-postgresql-flexible-server
- There might be limitations or issues with the service API or network issue in the West Europe region at that particular time that are not present in other regions - https://github.com/hashicorp/terraform-provider-azurerm/issues/24349
- You can choose to use Azure CLI for upgrade and/or review this for more solutions - https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-major-version-upgrade
I hope this is helpful! Do not hesitate to let me know if you have any other questions.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful